1. Field of the Invention
This invention relates to a demodulator and a communication apparatus, and more particularly to a demodulator and a communication apparatus wherein a DC (direct current) voltage of a demodulation signal produced in response to a phase difference between a local oscillation signal and a modulation signal synchronized in frequency with the local oscillation signal is detected and correction is carried out so that the detected DC voltage becomes equal to a reference voltage set in advance.
2. Description of the Related Art
A direct conversion method is known as a method of converting a high frequency signal of a carrier frequency directly into a baseband signal. Here, a communication apparatus of a popular direct conversion method is described. The communication apparatus includes a modulator and a demodulator. In the modulator, a modulation object signal is amplified by an amplifier and inputted to a mixer, and the modulation object signal and a local oscillation signal are multiplied by the mixer to produce a modulation signal. The amplifier amplifies the modulation signal produced by the mixer and transmits the produced modulation signal from an antenna to the demodulator.
On the other hand, in the demodulator, the modulation signal transmitted from the modulator is received by an antenna and amplified by an amplifier and then inputted to a mixer. The mixer multiplies the modulation signal received by the antenna and a local oscillation signal on the demodulator side to obtain a demodulation signal.
At this time, a frequency synchronization section in the form of a PLL circuit provided in the demodulator synchronizes the frequency of the local oscillation signal on the demodulator side with the frequency of the local oscillation signal on the modulator side so that the frequencies of the local oscillation signals on the modulator side and the demodulator side are set equal to each other. By the countermeasure described, a demodulation process with a high degree of accuracy can be achieved.
Here, frequency allocations in a case wherein frequency synchronization of a local oscillation signal is carried out and frequency allocations in another case wherein frequency, synchronization of a local oscillation signal is not carried out are described. FIG. 17A illustrates a frequency allocation of a local oscillation signal and a modulation signal on the modulator side where frequency synchronization of the local oscillation signal is carried out, and FIG. 17B illustrates a frequency allocation of a local oscillation signal and a modulation signal on the modulator side where frequency synchronization of the local oscillation signal is not carried out. FIG. 17C illustrates a frequency allocation of a demodulation signal where frequency synchronization is carried out, and FIG. 17D illustrates a frequency allocation of a demodulation signal where frequency synchronization is not carried out.
If a demodulation signal is produced where frequency synchronization is not carried out, then an offset of a frequency which corresponds to a frequency error between the local oscillation signal on the modulator side and the local oscillation signal on the demodulator side appears as seen from FIGS. 17B and 17D. As a result, a problem arises that the quality of the demodulation signal is deteriorated significantly. On the other hand, where frequency synchronization is carried out, the frequencies of the local oscillation signals of the modulator side and the demodulator side are held in synchronism with each other as seen in FIGS. 17A and 17C. Consequently, the demodulator side can obtain a demodulation signal of a desired frequency.
Incidentally, in recent years, communication apparatus which use a frequency of a millimeter waveband exceeding 30 GHz have been developed. However, if a frequency of a millimeter waveband is used in a communication apparatus which includes a PLL circuit described above, then there is a problem that a phase synchronization circuit such as a PLL circuit used for frequency synchronization has a comparatively great size. Further, where a frequency of a millimeter waveband is used, it is not easy to maintain the frequency dividing performance, the isolation performance and so forth of the PLL circuit. If it is tried to enhance the frequency accuracy in order to maintain the performances, then this gives rise to increase of the circuit scale and increase of the cost by the adjustment function. On the contrary, if the circuit scale is reduced, then this gives rise to a problem that the frequency error increases, resulting in degradation of the performances of the PLL circuit and so forth.
Thus, a communication apparatus of the injection lock system which carries out frequency synchronization without using a frequency synchronization circuit has been proposed. The injection lock system is a system which injects a frequency component of a local oscillator, that is, a local oscillation signal, on the modulator side into a local oscillator on the demodulator side to synchronize the frequency using the local oscillation itself.
FIG. 18 shows an example of a configuration of a demodulator 200 of the injection lock system, and FIG. 19 shows waveforms of a demodulation signal obtained by the demodulator 200. A modulation signal and a local oscillation signal are transmitted simultaneously from a modulator not shown. The demodulator 200 distributes and injects a modulation signal SRF amplified by an amplifier 202 into a local oscillator 206. The local oscillator 206 synchronizes the local oscillation frequency of the demodulator side with a carrier component of the modulation signal SRF of the modulator side injected thereto to produce a local oscillation signal SLO and outputs the local oscillation signal SLO to a mixer 204. The mixer 204 multiplies the local oscillation signal SLO inputted thereto and the modulation signal SRF to produce a demodulation signal. The produced demodulation signal is amplified by an amplifier 208 and then outputted.
As a communication apparatus which adopts the injection lock system, a communication apparatus has been proposed and is disclosed, for example, in Japanese Patent Laid-Open No. 2005-295594 (hereinafter referred to as Patent Document 1) wherein a reception side local oscillation wave is reproduced in synchronism with a low frequency local frequency signal wave included in a signal wave transmitted from a modulator. With this communication apparatus, the frequency of a signal wave can be converted with a high degree of accuracy by use of the reproduced reception side local oscillation wave.